As a process for producing hydrocarbons that can be used as feedstocks for liquid fuel products such as naphtha (raw gasoline), kerosene and gas oil, a process that employs a Fischer-Tropsch synthesis reaction (hereafter abbreviated as “FT synthesis reaction”) which uses carbon monoxide gas (CO) and hydrogen gas (H2) as a feedstock is already known.
Further, as a technology for producing liquid fuel base stocks from a gaseous hydrocarbon such as natural gas using the FT synthesis reaction, GTL (Gas To Liquids) Technology has been known. In this GTL Technology, a gaseous hydrocarbon such as natural gas is reformed to produce a synthesis gas containing carbon monoxide gas and hydrogen gas as main components, the synthesis gas is then subjected to the FT synthesis reaction to synthesize hydrocarbon compounds which are a mixture of hydrocarbons having a wide carbon number distribution, and further, the hydrocarbon compounds are hydroprocessed and fractionally distilled to produce hydrocarbon oils used for liquid fuel base stocks. According to the GTL Technology, liquid fuels containing substantially no environmentally hazardous substances such as sulfur compounds and aromatic hydrocarbons can be produced.
As the process for synthesizing hydrocarbon compounds via the FT synthesis reaction, a process in which the FT synthesis reaction is conducted by blowing the synthesis gas into a catalyst slurry prepared by suspending catalyst particles within a liquid hydrocarbon has been disclosed (see Patent Document 1).
In liquid fuel synthesizing systems that utilize the FT synthesis reaction for performing the aforementioned GTL Technology, the hydrocarbon compounds produced by the FT synthesis reaction is fractionally distilled, yielding a raw naphtha fraction, a raw middle distillate and a raw wax fraction. In this description, “raw naphtha fraction”, “raw middle distillate” and “raw wax fraction” mean respectively each of the fractions that has not been subjected to hydroprocessing (hydrotreating or hydrocracking)
In the FT synthesis reaction, besides the targeted paraffinic hydrocarbons, olefins and oxygen-containing compounds such as alcohols are produced as by-products. These by-products are impurities, and their inclusion within the liquid fuel products is undesirable. Accordingly, in an upgrading step, which composes a liquid fuel synthesizing system and performs hydroprocessing and fractional distillation of the raw naphtha, raw middle distillate and raw wax fraction obtained from the FT synthesis reaction to produce the fuel base stocks, the structures of the hydrocarbons that constitute each of the above fractions are transformed as required, and at the same time, the above impurities contained within each of the fractions are removed. In other words, the raw naphtha fraction is subjected to hydrotreating, the raw middle distillate is subjected to hydrotreating that includes hydroisomerization, and the raw wax fraction is subjected to hydrocracking Of the various fractions constituting the hydrocarbon compounds obtained from the FT synthesis reaction, the raw naphtha fraction contains the highest concentration of the olefins and alcohols.
In the hydrotreating of the naphtha fraction, the olefins and oxygen-containing compounds such as alcohols contained within the raw naphtha fraction are removed by a hydrogenation reaction and hydrodeoxygenation reaction respectively. Because these reactions are highly exothermic, excessive temperature increase in the naphtha fraction hydrotreating reactor is a concern. Accordingly, a portion of the inactive naphtha fraction which has been hydrotreated in the naphtha fraction hydrotreating reactor (hereafter referred to as the “treated naphtha fraction”) is typically returned to a point upstream from the naphtha fraction hydrotreating reactor, so that the freshly supplied raw naphtha fraction is diluted by this treated naphtha fraction before being supplied to the naphtha fraction hydrotreating reactor, and as a result, the excessive temperature increases in the reactor can be suppressed (see Patent Document 2).
On the other hand, in the hydrotreating of the naphtha fraction, the degree of progression of the above reactions has typically been controlled by adjusting the reaction temperature. Specifically, the treated naphtha fraction (in some cases, together with the raw naphtha fraction) is sampled and analyzed, and the residual concentration levels of the olefins and alcohols and the like within the treated naphtha, and/or the conversion thereof, are determined. Then, based on those results, the hydrotreating temperature (reaction temperature) is adjusted, and operations are controlled so as to achieve substantially no residual olefins and alcohols and the like within the treated naphtha.